Interferometers have found many uses in precision motion control due to their high accuracy and sensitivity. In applications requiring these qualities, an interferometer compares the phase of a laser reflected from a mirror mounted on a moving surface (such as a translation stage) to a fixed phase reference to extract the motion of the moving surface with respect to the reference. Because the wavelength of light used for the interferometric measurement is around 0.5 microns, interferometers can achieve sub-micron (and in some cases sub-nanometer) accuracy. Conventional interferometers compare the phase of a retroreflected laser beam with respect to a stationary reference beam in order to measure motion. However, some types of applications, such as scanning photolithography, require the synchronization of parts that move parallel to each other (e.g., a mask and a wafer) while interacting with an imaging system that is fixed with respect to the world coordinates. In this case, the quantity to measure is the relative motion of the mask image with respect to the wafer, rather than the relative motion of the mask and wafer. In order to accurately perform this task, it is necessary for the laser beam reflected from the object to go through the same imaging optical path as does the actual mask exposure beam.